1. Field of the Invention
The present invention relates generally to a reverse power control method and apparatus in a mobile communication system, and in particular, to a method and apparatus for controlling the transmission power of a reverse pilot channel to improve reverse traffic reception performance of a base station in connection with a data rate of a reverse traffic channel in a mobile communication system in which a mobile station determines a data rate of the reverse traffic channel.
2. Description of the Related Art
Typical mobile communication systems can be classified into a system for supporting a voice service and a system for supporting a data service. A typical example of such systems includes a Code Division Multiple Access (CDMA) system. A current CDMA system supporting only a voice service follows Interim Standard-95 (IS-95). With the progress of communication technology, mobile communication systems are being developed to support high-speed data services. For example, a first generation CDMA2000 (referred to as CDMA2000 1x) standard has been proposed which supports both the voice service and the data service, and a 1xEVDO (Evolution in Data Only) standard has been proposed which supports only a high-speed data service by assigning all possible resources to a data service based on a CDMA2000 1x system.
Signal transmission paths in a mobile communication system are generally classified into a forward path, i.e. a link in a direction from a base station covering a predetermined area (referred to as a cell) to a mobile station, and a reverse path, i.e. a link in a direction from a mobile station to a base station. In addition, a mobile station can exchange reverse/forward data with a base station while moving between cells.
A signal transmitted in a reverse direction by a mobile station includes a reverse traffic channel (R-TRCH), a reverse pilot channel (R-PICH), and various control channels. The reverse traffic channel has a variable data rate according to the amount of traffic transmitted thereon. The reverse pilot channel transmits control signals for channel compensation (i.e., channel compensation) and power control on a traffic channel, and is provided with a level of transmission power which is generally in proportion to a data rate of the traffic channel. As a data rate of a reverse traffic channel becomes greater, a base station needs to receive a pilot signal having higher power in order to perform smooth channel compensation on the traffic channel. Here, “channel compensation” refers to an operation in which a base station receiver compensates for an influence of a radio channel from a received signal so that the received signal can be demodulated.
A base station measures a received power to noise ratio of a reverse pilot channel, and controls the transmission power of a mobile station according to the measurement result. Such a transmission power control on a mobile station is performed in order to maximize reverse system capacity by minimizing interference occurring when a transmission signal transmitted by one mobile station affects a reverse signal from another mobile station, while maintaining reception performance of a reverse traffic channel.
Power control on a reverse signal performed in a CDMA2000 1x mobile communication system can be classified into an inner loop power control and an outer loop power control. In the inner loop power control, a base station transmits a power control bit (PCB) to a mobile station every time slot (1.25 ms) to control transmission power of the mobile station so that a received pilot energy to noise ratio Ep/Nt of a signal transmitted by the mobile station approaches a power control target setpoint. In the outer loop power control, the setpoint is adjusted every frame. The outer loop power control is used to maintain reception performance of a received traffic channel by adjusting the power control target setpoint.
In an exemplary method for performing the outer loop power control, if an error occurs in data received over a reverse traffic channel, the setpoint is increased by xdB, and if no errors occur, the setpoint is decreased by x/(1/FER−1) dB. FER denotes a frame error rate desired by a base station, and has a value between 0 and 1. For example, when x has a value of 1 and a desired error rate is 0.01, if an error occurs in reception data on a traffic channel, the setpoint is increased by 1 dB, and otherwise, the setpoint is reduced by 1/99 dB.
In addition to the outer loop power control another criterion for adjusting a power control target setpoint in a base station is a pilot reference level which is mapped to a rate of data transmitted by a mobile station. In the CDMA2000 1x standard, pilot reference levels shown in Table 1 below are defined according to data rates, and when a mobile station changes its reverse data rate, a base station adjusts a power control target setpoint accordingly.
TABLE 1Reverse Data RatePilot Reference Level 9.6 kbps0 19.2 kbps1 38.4 kbps11 76.8 kbps21153.6 kbps36307.2 kbps54
Shown in Table 1 are pilot reference levels mapped to corresponding data rates, defined in the CDMA2000 1x standard. The pilot reference level for each data rate specified in Table 1 is a value in a unit of 0.125 dB. For example, a pilot reverence level ‘x’ actually indicates x÷8 dB.
FIG. 1 is a diagram illustrating a power control method performed in a general CDMA2000 1x mobile communication system. Referring to FIG. 1, a base station instructs a mobile station to increase a data rate of an (i+3)th frame from 38.4 kbps to 76.8 kbps (Step 110). In this case, the base station increases a power control target setpoint from the (i+3)th frame in order to increase a reverse data rate. A level by which the power control target setpoint is to be increased is determined by the pilot reference levels specified in Table 1. Because a pilot reference level for 38.4 kbps is 11 and a pilot reference level for 76.8 kbps is 21 in Table 1, the base station increases the power control target setpoint by 1.25 (=10/8) dB corresponding to a difference value 10 between the pilot reference levels 11 and 21. Here, ⅛ dB represents a minimum unit by which the power control target setpoint is adjusted.
In the CDMA2000 1x mobile communication system, because a data rate of a mobile station is adjusted under control of a base station as stated above, the base station increases (up-adjusts) or decreases (down-adjusts) the power control target setpoint before the mobile station changes its data rate. The base station adjusts the power control target setpoint before the data rate is actually changed, which is done in order to maintain reception performance at the changed new data rate.
However, unlike the CDMA2000 1x mobile communication system, a 1xEVDO mobile communication system cannot adjust the power control target setpoint before a change in the data rate because, in the 1xEVDO mobile communication system, a data rate of a traffic channel transmitted by a mobile station is determined, not by a base station, but by the mobile station. In this case, the base station must adjust the related power control target setpoint after the mobile station changes its data rate.
FIG. 2 is a diagram illustrating the adjusting of a power control target setpoint in a base station at the change in a data rate of a reverse traffic channel in a general mobile communication system. Referring to FIG. 2, a mobile station uses a data rate of 38.4 kbps at an ith frame, increases the data rate to 76.8 bkps at an (i+1)th frame (Step 210), and increases the data rate to 153.6 kbps at an (i+2)th frame (Step 220) and maintains the data rate of 153.6 kbps at an (i+3)th frame. The data rates at the (i+1)th frame and the (i+2)th frame are increased compared with the data rates at their previous frames. Although the mobile station has increased the data rate at the (i+1)th frame compared with that of the previous frame in the manner stated above, a base station can identify the increase in the data rate after the base station receives a reverse rate indicator (RRI) over an RRI channel transmitted together with traffic data at the (i+1)th frame.
The RRI delivers information (traffic control information) related to a data rate, the number of transmission bits, a modulation scheme and a coding scheme of traffic data transmitted in the same time period (i.e., frame), to the base station. The reason why the mobile station transmits the RRI is as follows. In the 1xEVDO mobile communication system, because a data rate of a traffic channel is determined, not by a base station, but by a mobile station, the base station must be informed of the reverse traffic control information as stated above in order to receive reverse traffic data. That is, the base station can detect a rate of the traffic data transmitted by the mobile station at the (i+1)th frame after it receives the (i+1)th frame data.
In this case, the base station can adjust the power control target setpoint considering a data rate of a traffic channel, used at the (i+1)th frame, after the passage of the (i+1)th frame. That is, the power control target setpoint used for controlling power of the (i+1)th frame is not for the (i+1)th frame (76.8 kbps) but for the ith frame (38.4 kbps) or the previous frame.
When the power control target setpoint is not adjusted for at the point where a data rate is increased from 38.4 kbps to 76.8 kbps, like the (i+1)th frame of FIG. 2, reception performance of a traffic channel is deteriorated for the following reason. Since the power control target setpoint corresponding to the previous low data rate is used, although the data rate was increased, a reverse pilot signal having appropriate power to receive data on a traffic channel that uses the increased data rate is not received at the base station. The reverse pilot signal is used for channel compensation on the traffic channel, and in this case, the base station cannot secure a reception power estimation value sufficient for channel compensation, and also cannot obtain desired reception performance because reception power of the traffic channel is also less than an appropriate level.
Even when the data rate is increased to 153.6 kbps at, for example, the (i+2)th frame, the base station can set a power control target setpoint for a data rate of 153.6 kbps after it receives RRI transmitted by the mobile station at the (i+2)th frame. Because a data rate used at the (i+3)th frame is identical to a data rate used at the previous frame or the (i+2)th frame, the base station performs reverse power control using a power control target setpoint appropriate for receiving a frame at 153.6 kbps.
Comparing power of the (i+2)th frame with power of the (i+3)th frame in FIG. 2, it is noted that the (i+3)th frame is transmitted at higher power although it has the same data rate as the (i+2)th frame, for the following reason. That is, while a power control target setpoint used for controlling the transmission power of a mobile station at the (i+3)th frame was for 153.6 kbps, a power control target setpoint used for controlling the transmission power of the mobile station at the (i+2)th frame is for 76.8 kbps. Therefore, the base station can receive signals on a traffic channel and a pilot channel at the (i+3)th frame at sufficient reception power, but it cannot receive signals on the traffic channel and the pilot channel at the (i+2)th frame at sufficient reception power.
As described above, in the 1xEVDO mobile communication system in which a mobile station determines a data rate of a reverse traffic channel, a one-frame delay occurs in controlling a power control target setpoint considering a data rate of a reverse traffic channel. In this case, the base station's reception power for maintaining reception performance is not sufficiently secured, causing deterioration in the reception performance.